Fly ash, a by-product of coal burning power plant, is produced worldwide in large quantities each year. In 1988, approximately 84 million tons of coal ash were produced in the U.S. in the form of fly ash (60.7%), bottom ash (16.7%), boiler slag (5.9%), and flue gas desulfurization (16.7%) (Tyson, 1990, Coal Combustion By-Product Utilization Seminar, Pittsburgh, 15 pp.). Out of the approximately 50 million tons of fly ash generated annually, only about 10 percent is used in concrete (ACI Committee 226, 1987, "Use of Fly Ash In Concrete," ACI 226.3R-87, ACI J. Proceedings 84:381-409) while the remaining portion is mostly disposed of as waste in landfills.
It is generally more beneficial for a utility to sell its ash, even at low or subsidized prices, rather than to dispose of it in a landfill, since this will avoid the disposal cost. In the 1960's and 70's the cost of ash disposal was typically less than $1.00 per ton. However, due to the more stringent environmental regulations starting in the late 1970's, the cost of ash disposal has rapidly increased to from $2.00 to $5.00 per ton and is still rising higher (Bahor and Golden, 1984, Proceedings, 2nd International Conference on Ash Technology and Marketing, London, pp. 133-136). The shortage of landfill due to environmental concerns has further escalated the disposal cost. The Environmental Protection Agency (EPA) estimated in 1987 that the total cost of waste disposal at coal fired power plants ranged from $11.00 to $20.00 per ton for fly ash and bottom ash (Courst, 1991, Proceedings: 9th Int'l 1.0 Ash Use Symposium, 1:21-1 to 21-10). This increasing trend of disposal cost has caused many concerns and researchers are urgently seeking means for better utilization of fly ash. One potential outlet for fly ash is incorporation in concrete or mortar mixtures.
Fly ash is used in concrete in two distinct ways, one as a replacement for cement and the other as a filler. The first use takes advantage of the pozzolan properties of fly ash, which, when it reacts with lime or calcium hydroxide, can enhance the strength of cementitious composites. However, fly ash is relatively inert and the increase in compressive strength can take up to 90 days to materialize. Also, since fly ash is just a by-product from the power industry, the quality of fly ash has always been a major concern to the end users in the concrete industry.
Incorporation of fly ash in concrete improves workability and thereby reduces the water requirement with respect to the conventional concrete. This is most beneficial where concrete is pumped into place. Among numerous other beneficial effects are reduced bleeding, reduced segregation, reduced permeability, increased plasticity, lowered heat of hydration, and increases setting times (ACI Committee 226, 1987, supra). The slump is higher when fly ash is used (Ukita et al., 1989, SP-114, American Concrete Institute. Detroit, pp.219-240).
However, the use of fly ash in concrete has many drawbacks. For example, addition of fly ash to concrete results in a product with low air entrainment and low early strength development.
As noted above, a critical drawback of the use of fly ash in concrete is that initially the fly ash significantly reduces the compressive strength of the concrete. Tests conducted by Ravindrarajah and Tam (1989, Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, SP-114, American Concrete Institute, Detroit, pp. 139-155) showed that the compressive strength of fly ash concrete at early ages are lower than those for the control concrete, which is a general property of concrete or mortar when fly ash is added. Most of the reported studies tend to show a lower concrete strength due to the presence of fly ash; none has yet suggested a solution to actually enhance the property of concrete economically. Yet, for fly ash to be used as a replacement for cement, it must be comparable to cement in terms of strength contribution at a point useful in construction. As a practical matter, this means that the fly ash concrete must reach an acceptable compressive strength within about 2 weeks.
Swamy (1984, Proceedings, 2nd Int'l Conference on Ash Technology and Marketing, London, pp. 359-367) showed that 30% replacement by weight, and inclusion of a high dose of a superplasticizer, yielded concrete with material properties and structural behavior almost identical to those of concrete of similar strength without fly ash. However, due to the high cost of superplasticizer, mix proportions were not economical.
Fly ashes from different sources may have different effect to concrete. The same fly ash may behave differently with portland cements of different types (Popovics, 1982, ACI J. Proceedings 79:43-49), since different types of portland cement (type I to V) have different chemical composition. Other factors relating to the effects of fly ash on concrete that are not presently understood are lime availability, the rate of solubility and reactivity of the glassy phase in different fly ash, and the proper mix proportion to ensure early strength development of fly ash concrete.
Fly ash particles are typically spherical, ranging in diameter from 1 to 150 microns (Berry and Malhotra, 1980, ACI J. Proceedings 77:59-73). Aitcin et al. (1986, Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, SP-91, American Concrete Institute, Detroit, pp. 91-113) showed that if the average diameters, D.sub.50, of fly ash are smaller, the surface area of the fly ash will be larger than those with larger average diameters.
Many factors affect the size or average diameter of fly ash, including storage conditions, ash collection processes, and combustion conditions. Combustion conditions are perhaps most important, because these determine whether carbon remains in the ash or if combustion is complete.
There are two main forms of combustion: dry bottom boiler combustion and wet bottom boiler combustion. The main difference between the two types of boiler is that wet bottom boilers reach the fusion temperature of ash, thus resulting in fly ash with greater glass characteristics.
There are generally two methods known to measure the fineness of fly ash. The first is by measuring the residue on the 45 micron (No. 325 sieve), which is the method used in the United States. The second method is the surface area method by air permeability test. Lane and Best (1982, Concrete Int'l: Design & Construction 4:81-92) suggested that 45 microns sieve residue is a consistent indicator of pozzolanic activity. For use in concrete or mortar, ASTM C 618 (1990, ASTM C 618-89a, Annual Book of ASTM Standards, Vol. 04.02) specifies that not more than 34% by weight of a given fly ash be retained on a 45 microns sieve. However, Ravina (1980, Cement and Concrete Research 10:573-580) reported that specific surface area provides a more accurate indicator of pozzolanic activity.
Research carried out by Ukita et al. (1989, supra) purported that as the percentage of finer particles, i.e., those particles ranging from diameters of 1 to 20 microns, in concrete increases, the corresponding strength gain is notable. Similar observations have been reported by Giergiczny and Werynska (1989, Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, SSP-114, American Concrete Institute, Detroit, pp. 97-115).
Both of the groups mentioned above describe results with fly ash of disparate characteristics and sources, but did not include controls for these variable. Thus, although the emphasis of these reports is on the performance of finer particle fly ashes, the variables introduced into the studies lead to reservations with respect to any conclusions that may be drawn. In particular, Ukita et al. (1989, supra) collected fly ash from different locations. However, an earlier report demonstrated that fly ashes collected from different locations have different chemical properties (Liskowitz et al., 1983, "Sorbate Characteristic of Fly Ash," Final Report. U.S. Dept. of Energy, Morgantown Energy Technology Center, p. 211 ). Giergiczny and Werynska (1989, supra) ground the original fly ash into different sizes. Grinding can add metal particles into the fly ash, and also tends to yield unnaturally shaped particles of fly ash. Thus, these reports fail to provide conclusive information about the effect of fine particle size on the properties imparted by fly ash.
Berry et al. (1989, Fly Ash, Silica Fume, Slag, and Natural Pozzolans in Concrete, SP-114, American concrete Institute, Detroit, pp. 241-273) studied the properties of fly ash with particle size smaller than 45 microns, so called "beneficiated" fly ash, in mortar. Fly ashes of this particle size showed improved pozzolanic activity, reduced water demand and enhanced ability to reduce alkaliaggregate reactivity.
Although beneficiated fly ash seem to show promising results in terms of improved performance of mortar, other researchers concluded otherwise when used in concrete. Giaccio and Malhotra (1988, Cement, Concrete, and Aggregates 10:88-95) also conducted the test using the beneficiated fly ashes. They showed that the concrete made with ASTM type I cement, the use of beneficiated fly ash and condensed silica fume did little to enhance the properties of concrete compared with the raw fly ash.
It is critically important in construction to have concrete or mortar that predictably achieves required performance characteristics, e.g., a minimum compressive strength within 14 days. A corollary is that a construction or civil engineer must be able to predict the compressive strength of a concrete or mortar mixture after a given period of time. However, the prior art concrete or mortar mixtures that contain fly ash lack predictability with respect to compressive strength, and generally have lower compressive strength than concrete or mortar mixtures that lack fly ash. Therefore, there has been a disincentive to use fly ash in such hardenable mixtures.
Thus, there is a need in the art for a method of quantitatively determining the rate of strength gain of a concrete or mortar containing fly ash.
There is a further need in the art for high strength concrete and mortar containing fly ash.
There is yet a further need in the art for the utilization of fly ash generated during coal combustion.
The citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.